ViewPageTarget: Bind callback in onSerialize like in onShowChanges

init.mw.Target#serialize is an odd one because it is the only
target method that has a callback parameter (instead of
emitting the event on Target and having ViewPageTarget get
the event through that).

This line from onShowChanges needed to be added to onSerialize

  // Invalidate the viewer wikitext on next change
  this.surface.getModel().connect( this, { 'transact': 'onSurfaceModelTransact' } );

Though onSerialize is currently bound manually from the
serialize callback, it should be refactored to be like the others.

Bug: 44446
Change-Id: I9eddebbdf9294ee3d46286bdf1b157e00252d300